The present invention relates to switching voltage regulator circuits. More particularly, the present invention relates to switching voltage regulator circuits that use magnetic flux-sensing to regulate the output voltage of the regulator circuit.
The function of a voltage regulator is to provide a predetermined and substantially constant output voltage from an unregulated input voltage. Switching regulator circuits (hereinafter "switching regulators") typically use a power transistor as a switch to provide a pulsed flow of current to a network of inductive and capacitive energy storage elements which smooth the switched current pulses into a continuous and regulated output voltage. Switching regulators can provide output voltages which are less than, greater than, or of opposite polarity to the unregulated input voltage, depending on the mode of operation of the switching regulator. They often can be implemented to a large extent using integrated circuit components, which advantageously reduce the size and complexity of the overall switching regulator circuit.
Switching regulators are commonly used in power supply circuits. Switching-regulator power supplies generally can be classified into three categories, depending on the type of output circuitry used. These classes are: (A) single-ended inductor circuits, (B) diode-capacitor circuits, and (C) transformer-coupled circuits. A specific type of transformer-coupled switching regulator power supply is the offline power supply. Offline power supply circuits generate one or more DC output voltages from an AC input signal. The most desirable form of the offline power supply is known as a universal supply, which can accept an input ranging from 85 VAC to 270 VAC and therefore can operate with various power systems around the world without alterations or switches.
Commercially sold offline power supplies generally must provide DC outputs that comply with regulation and isolation requirements defined by various worldwide safety/regulatory agencies. Of particular importance are standards set by Verband Deutscher Elecktrotechniker, commonly referred to as the "VDE specifications." The VDE specifications, which have their origin in Germany, are generally regarded as the most stringent in the world. Therefore, a power supply which meets the VDE specifications usually will meet applicable safety regulation codes anywhere in the world.
The output voltage of an offline power supply is usually regulated by a feedback signal indicative of the output voltage. The feedback signal is used to control the duty cycle of the switching power transistor. There are various ways to provide such a feedback signal. One way is to sample the output voltage directly (e.g., by using a resistor network). This method provides the most accurate output voltage regulation. The problem with this type of feedback mechanism is that there is no isolation between the input and output circuits of the power supply, which may be operating at significantly different voltage levels. An opto-coupler can be used in the feedback signal path to provide isolation, but at the expense of numerous discrete components, large board space requirements, high cost, design problems associated with loop stability, spurious noise pick-up, start-up problems and output overshoot. Furthermore, opto-couplers age and thus this may cause loop response problems or loss of output regulation.
An alternate and simpler method for providing a feedback signal while maintaining isolation between the input and output circuits of the power supply is to employ what is commonly referred to as magnetic flux-sensing. Magnetic flux sensing is the generation of a feedback signal by sensing the magnetic flux in an inductive element (e.g., by sensing the flux in the core of a transformer). Magnetic flux sensing can be used in offline power supplies configured in a flyback converter topology, and may be used in other topologies as well. In a flyback converter topology, the flyback voltage on the primary winding side of the transformer during "switch off" time is sensed and used to regulate the output voltage. Ideally, the output voltage (VOUT) is related to the primary flyback voltage (VPRI) during "switch off" time by the relationship: EQU VPRI=(VOUT+Vf)/N
where Vf is the forward voltage of an output diode usually connected in series with the secondary winding and N is the transformer turns ratio from secondary to primary.
It is difficult to derive a feedback signal directly from the primary flyback voltage because this voltage is typically several hundred volts. A common practice is to provide a lower voltage auxiliary (or bias) winding from which the feedback signal is generated. In addition, this bias winding is convenient to use as a power winding for powering circuitry of the switching regulator circuit or other circuitry.
Magnetic flux-sensing simplifies the design of offline power supplies by minimizing the total number of components required while providing isolation between the input and output circuitry. Improved isolation can result in greater safety and reliability. However, although magnetic flux-sensing has been used in the past, regulator circuits incorporating it have had poor output voltage regulation due to the parasitics inherently present in a transformer-coupled design. These parasitics are especially troublesome in transformers which are designed to meet stringent safety and isolation requirements of certain industry standards, such as the above-discussed VDE specifications.
In view of the foregoing, it would be desirable to be able to provide a transformer-coupled switching voltage regulator circuit which uses magnetic flux sensing and which does not suffer from poor output voltage regulation due to the parasitics inherently present in a transformer-coupled design.